Aluminides crystal structures

Rogachev, A. S., Khomenko, I. O., Varma, A., Merzhanov, A. G., and Ponomarev, V. I., The mechanism of self-propagating high-temperature synthesis of nickel aluminides. Part II Crystal structure formation in a combustion wave. Int. J. SHS, 3,239 (1994a). 

The present monograph was first written as a chapter for Volume 8 of the series Materials Sdence and Technology A Comprehensive Treatment , edited by Robert W. Cahn, Peter Haasen, and Edward J. Kramer (Volume Editor Dr. Karl Heinz Matucha). Its aim is to give an overview of intermetallics, which is both detailed and comprehensive and which includes the fundamentals as well as applications. The result is an extended, critical review of the whole field of intermetallics with an emphasis on those intermetallic phases which have already been applied as functional or structural materials or which are currently the subject of materials developments. A historical introduction and a discussion of the relationship between atomic bonding, crystal structure, phase stability and properties is followed by a discussion of the major classes of intermetallics. The titanium aluminides, nickel aluminides, iron aluminides, copper phases, A15 phases. Laves phases, beryllides, rare earth phases, and siliddes are reviewed. In particular, the crystal structures, phase diagrams, and physical properties as well as the mechanical and corrosion behavior are treated. The state of developments as well as prospects and problems are discussed in view of present and future applications. The publisher has decided to publish the review as a separate monograph in order to make it accessible to a wider audience. 

At higher niobium levels, the 02 phase evolves to a new ordered orthorhombic structure that is based on the composition Ti2AlNb (O phase), which has been observed in titanium aluminides with compositions finm Ti-25Al-12.5Nb to Ti-25A1-30Nb (Ref 1-4). The crystal structures of the o2 and ordered orthorhombic phases are compared in the accompanying figure, which shows the basal planes and atomic positions in the lanes above and below the plane of sheet. 

The titanium aluminide Ti2AlNb with an ordered orthorhombic crystal structure rather than the ordered hexagonal DOiq structure of Ti3Al was stronger and has higher fractvire toughness than... 

The melting point of titanium is 1670°C, while that of aluminium is 660°C.142 In kelvins, these are 1943 K and 933 K, respectively. Thus, the temperature 625°C (898 K) amounts to 0.46 7melting of titanium and 0.96 melting of aluminium. Hence, at this temperature the aluminium atoms may be expected to be much more mobile in the crystal lattices of the titanium aluminides than the titanium atoms. This appears to be the case even with the Ti3Al intermetallic compound. The duplex structure of the Ti3Al layer in the Ti-TiAl diffusion couple (see Fig. 5.13 in Ref. 66) provides evidence that aluminium is the main diffusant. Otherwise, its microstructure would be homogeneous. This point will be explained in more detail in the next chapter devoted to the consideration of growth kinetics of the same compound layer in various reaction couples of a multiphase binary system. 

The titanium aluminide TiAl - often designated as y phase - crystallizes with the tetragonal LIq structure (CuAu-type) which is shown in Fig. 1. The LI o structure results from ordering in the f.c.c. lattice (Al), i.e. it is basically a cubic structure which is tetragonally distorted because of the particular stacking of the atom planes, as is seen in Fig. 1. The ratio of the lattice parameters c and a is cja = 1.015 at the stoichiometric composition and the density is 3.76 g/cm (Kim and Dimiduk, 1991), whereas for TiAl-base alloys the range 3.7-3.9 g/cm is given (see Table 2). This density is still lower than that of TijAl and has made the titanium aluminides most attractive for materials developments. 

The nickel aluminide NijAl - known as the y phase - crystallizes with the cubic LI2 structure (CujAu-type) which results from the fc.c. structure by ordering (see Fig. 1). Deviations from stoichiometry are accommodated primarily by antisite defects (Lin and Sun, 1993). The density of NijAl is 7.50 g/cm (see Liu et al., 1990) and thus is only slightly lower than that of the superalloys (see Table 2) which, however, is still of interest. The elastic constants have been studied experimentally and theoretically by various authors (e.g. Davies and Stoloff, 1965 Dickson et al., 1969 Kayser and Stassis, 1969 Foiles and Daw, 1987 Wallow et al., 1987 Yoo and Fu, 1991, 1993 Yasuda et al., 1991a, 1992). Young s modulus of cast polycrystalline NijAl at room temperature is about the same as that of pure Ni with a weaker temperature dependence (Stoloff, 1989),... 

Figure 10.16 Optical micrograph showing the cross-section of a Pt-modified aluminide coating on a nickel-base single-crystal superalloy after oxidation at 1200 °C for 20 h. The original grain structure of the /3-phase is evident and y has begun to nucleate at /6 grain boundaries as a consequence of A1 depletion.

The aluminides with nickel and copper as transition-metal component show a very interesting behavior. At normal pressure conditions, YbNiAl (Schank et al. 1995, Rossi et al. 1983a,b) and YbCuAl (Mattens et al. 1982) crystallize with the hexagonal ZrNiAl type (Krypyakevich et al. 1967, Jacob et al. 1987, Zumdick et al. 1999) (fig. 3), a ternary ordered version of the Fe2P structure (Rundqvist and Jellinek 1959) (space group P62m). 

With few exceptions boride structures are different from the structures of aluminides, silicides, and germanides. Aluminides often have the same structures as the silicides and germanides, but with T and M elements interchanged on their crystallographic sites. Ternary phases containing Sc generally crystallize in structure types which differ from those found with the other rare earths. Due to its relatively small size the element Sc behaves rather more similar to Zr and Hf in its ternary structures. 

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